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The bound unoccupied electronic state structure of an Ir(111)/graphene surface covered by differently sized and spaced Ir clusters was investigated by means of two-photon photoemission spectroscopy. The cluster lattice was found to affect the image potential states of the substrate to a surprisingly large extent. This effect can be related to the influence of the cluster lattice on the screening of the image state electron trapped in front of the surface. The symmetric arrangement of Ir clusters considerably reduces the lateral extension of graphene areas with a homogeneous local work function, and from a certain minimum area size, the excitation of an electron to a stable state in a Coulomb-like potential is not possible anymore. Furthermore, lateral confinement effects could be observed due to the decreasing extension of bare graphene areas. [ABSTRACT FROM AUTHOR]

Reactions of free silver anions Ag[sub n][sup -] (n=1–13) with O[sub 2], CO, and their mixtures are investigated in a temperature controlled radio frequency ion trap setup. Cluster anions Ag[sub n][sup -] (n=1–11) readily react with molecular oxygen to yield Ag[sub n]O[sub m][sup -] (m=2, 4, or 6) oxide products. In contrast, no reaction of the silver cluster anions with carbon monoxide is detected. However, if silver cluster anions are exposed to the mixture of O[sub 2] and CO, new reaction products and a pronounced, discontinuous size dependence in the reaction behavior is observed. In particular, coadsorption complexes Ag[sub n](CO)O[sub 2][sup -] are detected for cluster sizes with n=4 and 6 and, the most striking observation, in the case of the larger odd atom number clusters Ag[sub 7][sup -], Ag[sub 9][sup -], and Ag[sub 11][sup -], the oxide product concentration decreases while a reappearance of the bare metal cluster signal is observed. This leads to the conclusion that carbon monoxide reacts with the activated oxygen on these silver clusters and indicates the prevalence of a catalytic reaction cycle. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The oxidation of carbon monoxide with nitrous oxide on mass-selected Au3+ and Ag3+ clusters has been investigated under multicollision conditions in an octopole ion trap experiment. The comparative study reveals that for both gold and silver cations carbon dioxide is formed on the clusters. However, whereas in the case of Au3+ the cluster itself acts as reactive species that facilitates the formation of CO2 from N2O and CO, for silver the oxidized clusters Ag3Ox+ (n = 1-3) are identified as active in the CO oxidation reaction. Thus, in the case of the silver cluster cations N2O is dissociated and one oxygen atom is suggested to directly react with CO, whereas a second kind of oxygen strongly bound to silver is acting as a substrate for the reaction. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Free Mn x O x + (x = 3, 4) clusters mediate the decomposition of acetic acid molecules. • Decarboxylation of acetic acid to CO 2 in competition with the dehydration to ketene (in the case of Mn 3 O 3 +) are observed. • Major reaction channel, however, consisted in the deprotonation of the acetic acid. • Implications and perspectives for the selective catalytic ketene synthesis are discussed. Abstract The decomposition of acetic acid upon reaction with the free mass-selected manganese oxide clusters Mn 3 O 3 + and Mn 4 O 4 + was studied in an octopole ion trap under well-defined reaction conditions. Three distinct reaction channels have been identified: (1) the decarboxylation of acetic acid to CO 2 , (2) in the case of Mn 3 O 3 + the dehydration of acetic acid to ketene in competition with the decarboxylation, and (3) the deprotonation of the acid. The latter was found to represent the major reaction channel for both investigated cluster ions. The observation of the manganese oxide cluster mediated decomposition of acetic acid represents an important first step for future investigations aiming at the development of new materials for the selective catalytic ketene production. [ABSTRACT FROM AUTHOR]

Coadsorption effects of molecular hydrogen and small hydrocarbons, CH4 and C3H6, on free Au3 + and Au5 + were investigated in an octopole ion trap under multi-collision conditions. For hydrogen and methane the observations indicate that both molecules coadsorb on the same adsorption site, i.e., the same atom of the cluster. This type of molecular adsorption on free clusters is termed permissive coadsorption, in contrast to competitive coadsorption, in which two molecules compete for the same adsorption site. The latter case was observed for hydrogen and propylene: already trace amounts of propylene were able to completely saturate the clusters preventing the coadsorption of H2. The size dependent adsorbate coverage is discussed and implications on the cluster structure are deduced from time and temperature dependent reaction measurements. [ABSTRACT FROM AUTHOR]

Abstract: The reactions of free, size selected gold and silver cluster cations (n =3, 5) with C6H6 as well as with a mixture of C6H6 and CO were investigated in a radio frequency ion trap under multiple collision condition. While benzene was found to react with all investigated metal clusters exhibiting size dependent adsorbate coverages, the coadsorption of C6H6 and CO was only observed on the gold clusters. Photodissociation experiments at 353 and 393nm, respectively, provided indications for charge transfer induced fragmentation in the case of the silver cluster–benzene complexes. In particular, for the femtosecond time resolved fragmentation dynamics could be measured. [Copyright &y& Elsevier]

Abstract: Gas phase kinetics experiments with mass-selected gold, silver, and binary silver–gold cluster ions in a radio frequency (rf)-ion trap reactor are presented. For the catalytic CO combustion reaction, the adsorption of molecular oxygen onto the free clusters is identified as the first reaction step. A comparison of the measured O2 adsorption reaction rate constants for the investigated Ag n Au m − (n + m =1, 2, 3) clusters reveals a pronounced size and composition dependence. Favorable activation of the oxygen molecular bond is only expected in the case of Au2 −. In the reaction of the gold dimer with O2 and CO, a coadsorption complex is identified at cryogenic temperatures as decisive reaction intermediate in the observed CO oxidation reaction. Through detailed reaction kinetics measurements in combination with first-principles calculations, a comprehensive understanding of the molecular details of the catalytic reaction cycle emerges. The obtained data also permit the determination of the CO2 formation rate and the catalytic turn-over-frequency in the rf-ion trap reactor. In contrast to gold, odd size silver cluster anions are found to adsorb two oxygen molecules. These Ag n O4 − complexes are proposed to be key intermediates in oxidation reactions with silver clusters involved, and first experimental indications for catalytic activity of selected cluster sizes are presented. [Copyright &y& Elsevier]

The reactions of free gold clusters Au-, Au2-, and Au3- with carbon monoxide are studied in an rf-octopole ion trap experiment at cryogenic temperatures. While Au- is unreactive toward CO over the whole temperature range investigated, the two and three atom cluster anions show a maximum adsorption of two CO molecules at temperatures below 250 K. From time resolved trapping experiments the strongly cluster size dependent reaction kinetics are obtained and a distinct reaction mechanism is deduced. The size dependence of the measured rate coefficients reveals the preferred formation and particular stability of the carbonyl complex Au3(CO)2- . Through RRK analysis of the absolute termolecular rate coefficients we are able to estimate the binding energy of CO to Au2- and Au3-. [ABSTRACT FROM AUTHOR]

Abstract: The reactions of the mass-selected gas-phase gold cluster cations Au3 + and Au5 + with H2O, CH3Cl, and mixtures of these reactants were studied under well-defined multi-collision conditions in an octopole ion trap. The reaction of CH3Cl with the gold cations was found to proceed fast at room temperature, leading to the adsorption of one CH3Cl molecule at each ‘corner’ atom of the cluster ion. This strong adsorption hindered the coadsorption of other reactants like H2O. However, a considerable reduction of the CH3Cl partial pressure led to distinct patterns of H2O/CH3Cl coadsorption products. Furthermore, the mass spectra indicated the loss of hydrogen after the reaction of CH3Cl with Au3 +. [Copyright &y& Elsevier]

Temperature-dependent gas phase iontrap experiments performedunder multicollision conditions reveal a strongly size-dependent reactivityof Pdx+(x= 2–7) in the reaction with molecular oxygen. Yet, a particularstability and resistance to further oxidation is generally observedfor reaction products with two oxygen molecules, PdxO4+. Complementary first-principles densityfunctional theory simulations elucidate the details of the size-dependentbonding of oxygen to the small palladium clusters and are able toassign the pronounced occurrence of PdxO4+complexes to a dissociatively chemisorbedbridging oxygen atomic structure which impedes the chemisorption offurther oxygen molecules. The molecular physisorption of additionalO2is only observed at cryogenic temperatures. Additionalexperiments and simulations employing preoxidized clusters PdxO+(x= 2–8)and PdxO2+(x= 4–7) confirm the formation of the two differentoxygen species. [ABSTRACT FROM AUTHOR]

Thegas-phase reaction of size-selected Run(n= 4–6) clusters withCO in an ion trap yields only one specific ruthenium carbonyl complexfor each cluster size, Ru4(CO)14, Ru5(CO)16, and Ru6(CO)18. First-principles density functionaltheory calculations reveal structures for these hitherto unknown carbonylcompounds that are in perfect agreement with the geometries predictedby Wade’s electron counting rules. Furthermore, reactions withD2show that for Ru4橷 Ru6, CO molecules can be partially replaced by D2to form hydrido carbonyl complexes while preserving the totalligand count corresponding to the Wade cluster sizes. [ABSTRACT FROM AUTHOR]

Highlights: [•] The reactions of free gold–palladium clusters Au2Pd+, AuPd2 +, and Au2Pd2 + with methane have been investigated in an ion trap experiment. [•] Methane binding energies are determined. [•] AuPd2 + dehydrogenates methane and a mechanism for the catalytic ethylene formation is proposed. [•] The results are compared to the bare gold Au n + (n =2–4) and palladium Pd m + (m =2–4) clusters. [Copyright &y& Elsevier]